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I've read the OC3D review of Corsair AX850 that the unit still runs passively after 20 minutes of full load, which is what I want. But the report from Cybenetics shows the fan starts at around 400W load.
On the other hand Seasonic's fan starts a bit earlier but at lower RPM overall.

Most of the time I use my PC browsing the web, watching Youtube. Sometimes gaming and video/audio editing/rendering.

I'm currently using EVGA 850 G3 with ECO mode ON and it's the noisiest component. The PSU fan suddenly ramp up to quickly cool itself down after idling on desktop around 5-10 minutes and then back to passive mode and then ramp up again... repeat and repeat. Even with ECO mode off I still don't want that constant fan hum from the PSU. That's why I'm changing it.

For the small system you don't need 850W, take the Seasonic Prime Fanless and you don't have to worry about the fan.

In the OC3D review of Corsair AX850 the fan did not start because the power supply unit was tested outside the case and did not warm up.

I've used the PSU calculator and it says my load wattage is round 550W, I afraid I may actually able to reach such wattage while doing rendering. So I afraid the 600W fanless is not an option since I would like to keep some headroom for the PSU.

I've used the PSU calculator and it says my load wattage is round 550W,

They are lying to you and overestimating by a mile.
DO NOT Trust them!
Because CPU and GPU are the biggest Power consuming things...
The rest hardly matters...

Originally Posted by boveywong

I afraid I may actually able to reach such wattage while doing rendering. So I afraid the 600W fanless is not an option since I would like to keep some headroom for the PSU.

I doubt it.
Your System is probably nowhere near 500W...
And OC for a rendering machine? that is a bad idea s the OC increases the failure rates. And you might not want some problems with the rendering (and do it again), do you??

Anyway, you might want to get a Powermeter to measure the Power Consumption of your system for various reasons...

Thing is: 550W G3 would have been somewhat acceptable, the higher wattage are not...

I've read the OC3D review of Corsair AX850 that the unit still runs passively after 20 minutes of full load, which is what I want. But the report from Cybenetics shows the fan starts at around 400W load.

Because OC3D tests at room temperature and Cybenetics tests at 30 to 32°C.

I used to run my i7 8700K with all 6 cores at 5.0 GHz, Vcore=1.37. PSU was a 460w Seasonic Gold fanless. Admittedly there was no dGPU involved, but an OC'd 8700K had no problems with 460W. You should have no problem adding a GPU if your PSU is 600W.